In a holographic data storage system there are three or four independent optical channels depending on the system architecture: recording and/or reading reference beams and recording and/or reading object beams with spatial light modulator (SLM) and detector. In simple laboratory systems these optical channels use different optical paths, they are spatially separated from each other: overlapping occurs only in the storage material. This requires a high number of imaging objectives, mirrors, prisms, which leads to large dimensions and the need to adjust and control the optical channels independently. The independent paths are sensitive to environmental vibration. If the system size, i.e. the number of optical elements, is to be reduced, then the optical paths for the independent optical channels have to be used partially in common. To achieve this, specially designed optical elements are needed for coupling-in the independent channels after the laser and for coupling-out the independent channels before the detectors. The beams coupled together can go through the same optical paths and use the same objectives, which yields a so-called collinear arrangement. This arrangement is insensitive to environmental effects like vibration, air turbulence, temperature changes etc. Consequently, for a practically applicable holographic data storage system a collinear optical arrangement is favorable. Coupling-in and -out of the essentially parallel reference and objects beams can either be solved spatially, on the basis of polarization, or by utilizing the different beam convergence angles (numerical aperture, NA).
From the point of view of practical applicability it is desirable to use reflection-type holographic media, where all the optical elements are arranged on the same side of the holographic disc. When using transmission-type holographic media, parts of the optical system are arranged on opposite sides of the holographic disc. In this case two servo systems are needed for the optics on both sides, and/or large and heavy mechanical elements to maintain the optical elements in correct position relative to the surface of the holographic disc and relative to the tracks on both sides of the holographic disc. On the other hand, using reflection-type holographic media much simpler and smaller servos and mechanical elements can be employed.
In document EP1065658 a collinear holographic arrangement with polarization coupling-in and -out is used, combined with CD/DVD-like servo systems. According to the disclosed solution the more or less parallel object and reference beams are each cut into two orthogonally polarized half-cones. The orthogonally polarized half-cone reference beams and object beams are coupled-in and -out based on their polarization using special polarization rotating elements and polarization sensitive beam splitters. Depending on the optical arrangement and the material sensitivity, the orthogonally polarized beams form at least two or more micro-holograms. From a geometrical point of view these micro-holograms are arranged beside each other. However, taking into account diffraction of the object and reference beams, the micro-holograms overlap each other, and also overlap the oppositely polarized reference beams. During the reading process this overlapping generates unwanted ghost images and reduces the signal-to-noise ratio (SNR) of the read image. Consequently, using the polarization collinear holographic optical setup presented in EP1065658 it is problematic to filter out unwanted ghost images. The solution according to the present invention avoids the ghost images generated by the direct and reflected reference beams.
It is a cost-effective idea to use the well-known CD/DVD-like servos for holographic media. In these digital optical storage systems a “single point” of the disc surface is written or read using servos having only three degrees of freedom. In holographic storage information is stored in a three-dimensional volume requiring servos having six degrees of freedom. This means that for recording and reading the reference beams must be adjusted to the hologram in six degrees of freedom: for holographic storage it is not feasible to use the well-known track and focus servos. Depending on the optical system, due to the large NA of the reference and/or object beam additional servos must be introduced for tilt compensation of the holographic disc or the reference beam. EP1065658 does not describe any solution for compensating the tilting errors.
In a holographic system there is an object-image relation between the SLM and the detector array, and the “distance” between them is finite. In CD/DVD/HD-DVD/Blu-Ray systems the focusing objective is illuminated by a “plane wave” so the object and the image are at essentially infinite distances from each other, which is called infinite-conjugate imaging. In this case the coaxial movement of the objective during focus error compensation does not cause a blur of the imaged focus spot. In holographic systems the simple CD-like focus servo causes a blur because of the finite distance between object and image. In EP1065658 there is no solution for the compensation of the blur caused by movements of the objective during the focusing. The present invention suggests a technically viable solution to compensate for the movements of the imaging objective of a holographic storage system.